Trees are cultivated for timber production in two main ways; firstly an extensive form, where the timber goes to both sawmills and pulp and paper with growth time up to 100 years or more.
This is the dominating form of forestry in for instance Scandinavia and Canada. Secondly, trees can be cultured in more agricultural forms in plantations. This is common in many tropical and subtropical countries as Brazil and Indonesia. The trees (mostly hardwoods as Eucalyptus) might be harvested already after 5 to 7 years and are in some cases exclusively used for pulp and paper.
The following description is mainly about forestry according to the first form. A high ambi-tion in regeneraambi-tion, clearing and thinning is a prerequisite for a sustainable high yield of good quality timber from the forestland. In general, the forest industry wants straight trees of high density and with thin, frail branches.
The fibres created in the beginning of the growth season, earlywood fibres, have a lower den-sity compared to latewood fibres. Softwoods growing fast will form a higher proportion of ear-lywood fibres and thereby have a lower wood density. It is therefore desirable for softwoods to have a slow growth rate in order to have high quality wood. It is especially important for a slow growth rate during the first 20 years, in order to reduce the amount of juvenile wood with short-er fibres. By limiting the softwood seedlings access to nutrients, light and watshort-er the wood qual-ity can be improved. Many seedlings growing close to one another and competition from bigger trees will reduce the growth rate of the seedlings. Ring-porous hardwood, such as oak, ash, and elm, the opposite is true. The faster they grow, the more latewood is formed and the higher the density of the wood becomes. The diffuse-porous hardwoods, like birch and aspen, form vessels of more or less same size in the early and late growth season. The density of these trees is not affected by the growth rate.
Clearing the forest is to remove unwanted growth from it and gives ample possibilities to in-fluence the future stand. By clearing, suitable wood species and high quality stems are favoured.
In the subsequent thinning, the trees felled have coarser stems and will give more profit. In an uncleared stand, the trees become more slender and a slender tree costs as much to fell as a coarse tree, but gives less revenue. Not to clear is a waste of capital. On the other hand, if the clearing is too severe giving too much space to individual trees, these will have the coarsest branches and the highest amount of juvenile wood.
Thinning, also called intermediate cutting, can improve the quality of the remaining stand by proper selection of the trees to be felled. The thinning material is the main wood supply to the pulp and paper industry. In other words, pulpwood is primarily thinning material.
Fertilisers, mainly nitrogen, phosphorous and potassium (see 2.3.2), can be used to increase forest production. However, the high precipitation of nitrogen from fossil fuels makes it in most cases needless to add any more nitrogen. When the air polluting nitrogen and sulphuric oxides precipitate in the forest, they leach out alkaline cations, thus decreasing the pH of the soil. The removal of big amounts of biomass, when harvesting timber, disrupts the balance of the soil productivity. Lime or wood ashes can be supplied in order to counteract the acidifying effects of air pollutants. In Scandinavia fertilizing of forests are rare nowadays.
Moose, deer, and other game can cause damages on growing forest by grazing on shoots and branches, breaking stems and gnawing off the bark of trees. Hunting, to keep the number of game on a supportable level, is an important forestry measure. Game enclosures can be neces-sary in certain areas to keep grazing animals out. The grazing can also be manoeuvred to areas less sensitive, by providing special grazing pastures, supplying salt stones or supportive feed-ing. Chopping off hardwood shrubs about one meter above ground, gives new shoots and the game access to more feed.
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Harvesting can be done in two ways, either one by one when the trees are large enough, or a more or less complete harvesting. The earlier is often performed in small scaled with relatively simple equipment; horses are still used for transports (elephants in south Asia). The latter is usually performed with a harvester, which fells the tree, trims it and cuts it to desirable lengths, using a computerized cut-to-length system.
The forwarder takes the logs to the deposit where they are separated into different assort-ments. Trucks deliver the logs directly to the industry or to the railway for further transport to the industry. The time it takes to deliver timber from the forest to the industry is quite short. The pulping industry needs fresh raw material and the cost of storage is high. The best quality of the full-grown timber goes to the sawmills. Lower quality (damages, moderate rotted) goes to the pulp and paper industry as pulpwood as well as cleaning and thinning timber.
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After cutting the timber in an area, a new forest is usually established by planting. In the north-ern part of Sweden spruce and pine are planted to the same extent, whereas in southnorth-ern Sweden spruce plants dominate. Hardwood is primarily planted on abandoned arable land. Ground prep-aration is usually a prerequisite for a successful planting. The soil is clarified from the under-growth so that more light reaches the plant. The soil surface temperature is thereby increased leading to a decreased risk of frost killing the juvenile plants. Soil clarification also diminishes the competition from roots of other plants and favours the sowing of other tree species.
• Sowing seeds is only performed on a minor portion of the regeneration area. The probabil-ity of a successful sowing is higher for the northern parts of Sweden. In the more southern
parts, the temperature can alternate between a few degrees above and below freezing point, with a risk of freezing the small plant.
• Natural regeneration. This can be a good way to restore pine forest, providing it is a good seed year and required preparations have been made. Before cutting for timber, the best seed trees are selected (Figure 2.19) and left to provide for natural sowing. Soil clarifica-tion is generally necessary and should be performed on a year when there are plenty of seeds. Some 50–150 seed trees are required per hectare. Natural regeneration is as a rule the best for hardwoods. Birch can be regenerated by seeds as well as by stump shoots. Root shoots regenerate aspen. Hardwood regeneration generally requires game enclosure to fence out moose, deer etc that eat plants and shoots.
Figure 2.19. Natural regeneration. Some birch trees was saved after harvesting for naturally sawing new birch plants. Note that branches are left in the wood.
• Shelterwood offers many advantages for regeneration and can be used for softwood as well as hardwood. The function of the shelterwood is as seed trees and protection for the plants against competing shrubbery, frost damages and harmful insects. It has been shown that regeneration under high shelterwood decreases the attacks of pine weevil (Figure 2.20).
Figure 2.20. Shelterwood. Tall, storm resistant trees, mainly pine and occasionally hardwood, are chosen for high shelterwood. For low shelterwood, the hardwood appearing after harvesting timber is used to protect plants against frost.
high shelterwood low shelterwood
The above-mentioned methods are based on harvesting practically all the timber in the cut-ting area, except for seed trees or shelterwood. Other forestry methods are gaining interest, call-ing for more expert knowledge and long-term planncall-ing. They include for example edge fellcall-ing of spruce forest, in which 10–20 m wide rows are cut with 5–10 years intervals. The spruce trees along the rows will provide for the regeneration with their seeds. The age of the trees will be diverse row-wise in this kind of forest. Group selection cutting is a similar method, but in-stead of rows, circles of 20 m diameter are cut with 5–10 years interval. The openings are wid-ened by 10 m at a time until the circles merge. The age of the trees will be diverse group-wise.
In environmentally very sensitive areas, alternative forestry methods may be used, for example selective felling of trees with a certain diameter.
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As mentioned above plantations are a more agricultural type of forestry. The global trend is to-wards increased establishment of plantations. Some plantations are created in order to rehabili-tate a certain environment or for soil and water conservation, but many plantations are an important source of raw material for the pulp and paper industry. The plantations account for some 5 % of the total forest area of the world, Table 2.7. The countries with major industrial plantations are China (37 million ha), the United States (16 million ha), and India (12 mil-lion ha).
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Some countries, such as for example Chile and New Zealand, have established plantations on large areas. They are able to not only supply the domestic industry with wood raw material but also export significant amounts of wood.
The tree species most commonly planted are in the genera Pinus (specially Pinus radiata), Acasia and Eucalyptus. Thus they are introduced in regions, even continents far from their ori-gin32. Attacks by parasites (fungi or insects) that the trees have low resistance against are not unusual problems.
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32 Introduction of ”foreign” species are not restricted to plantation forestry. Pinus contorta from North America have been introduced in Scandinavian forests and different American hemlock species in the continental Europe.
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The World Conservation Union (IUCN) has put up a classification system for protected areas shown in Table 2.8. Categories I and II are dominated by environmental considerations and are to be left unattended by human impact except for scientific purposes. Categories III and IV need management to preserve the special the specific natural feature or habitat. Categories V and VI are dominated by production goals.
The environmental value of a forest depends to a great deal on how it can provide for biodi-versity. A variation in tree species grants a variation in species of other plants, insects, birds and mammals. However, natural monocultures of pine or spruce are biodiverse, if the age of the trees varies and many dead trees are available. Special landscape types, such as hillsides and streams, offer a habitat for a varied assortment of plants and animals. Some species are threat-ened by extinction, very rare or sensitive. They are denoted red-listed species and need special concern. In addition to environmental values, cultural values demand care. Examples are ar-chaeological sites and old summer farm holdings. Most forests fall into categories V or VI. As an example, only 4% of Sweden’s forestland is protected and excluded from wood production.
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The forests are decreased by harvesting roundwood as well as a result of forest fires etc. On the other hand, forest area increases, either by plantation or by natural growth of existing for-ests. Historically, the deforestation of our planet has been extensive and it still continues. Glob-ally, there is a loss of forest area by more than 9 million hectares annuGlob-ally, Table 2.9. However, as also seen from the table, the loss is of tropical forests, whereas in non-tropical areas there is an actual gain in forestland.
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Certification of the forestry signifies an agreement to follow certain obligations. It is a voluntary agreement with an independent party issuing the certificate and performing regular control. The certification provides the companies with an environmental management system with routines for an efficient and structured environmental work.
• FSC, Forest Stewardship Council, is an environmental management system with the aim to promote an environmentally sustainable forestry. These apply mostly for bigger multina-tional companies. As of 2001, some 22 million ha of forestland is certified by FSC.
• PEFC, Pan-European Forest Certification, and FFC, Family Forest Certification, aim to promote family forestry that adopts a responsible approach to environment as well as pro-duction and social standards.
• The International Organization for Standardization has an Environmental Management System, ISO 14001 and the EU provides a standard EMAS, Eco Management and Audit Scheme. These systems present a more general environmental management and do not pro-vide forest management certification
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Raven P.H., Evert R.F., and Eichhorn S.E. (1999) Biology of Plants, 6th edition. New York, NY, USA: WH Freeman and company Worth Publishers, ISBN 1-57259-041-6.
Kellomäki A. (1998) Forest resources and sustainable management. Helsinkki, Finland: Fapet Oy, ISBN 952-5216-02-0.
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Geoffrey Daniel
Swedish University of Agricultural Sciences (SLU), Department of Forest Products
3.1 Introduction 46
3.1.1 Formation of Wood Cells 47 3.2 Softwood and Hardwoods 48 3.2.1 Softwood Cell Types 48 3.2.3 Softwood Tracheids 49 3.2.4 Softwood Rays 51
3.2.5 Softwood Parenchyma Cells 51 3.2.6 Longitudinal Parenchyma 51 3.2.7 Epithelial Parenchyma 52 3.3 Resin Canals 52
3.4 Anatomy of Hardwoods 52 3.4.1 Hardwood Fibres 54 3.4.2 Parenchyma Cells 55
3.4.3 Longitudinal Parenchyma Cells 55 3.4.4 Ray Parenchyma 55
3.5 Hardwood Tyloses 56
3.6 Wood Pits: Simple, Bordered and Cross-field Pitting 57 3.6.1 Softwood Pits 58
3.6.2 Hardwood Pits 59
3.7 Wood Cell Wall Structure and Ultrastructure 60 3.7.1 Models of Wood Cell Wall Organization 60 3.7.2 Structure of Cell Wall Layers 61
3.7.3 Molecular Models of Cell Wall Ultrastructure 64 3.7.4 Chemical Composition of Different Cell Wall Layers 65 3.8 Reaction Wood 66
3.8.1 Morphology of Compression Wood Tracheids and Tension Wood Fibres 67 3.9 Methods for Studying Wood and Cell Wall Structure 68
3.10 References and Further Reading 69